Athermal acoustic lens

ABSTRACT

This invention is an acoustic refractor that contains a combination of individual fluid lens elements having different indices of acoustic refraction which cooperate to produce a uniform acoustic refraction over a wide range of ambient temperatures.

ilniied States Patent inventor Ernest A. i-iogge [56] References Cited 1N Egg City! UNITED STATES PATENTS P 3,483,504 12 1969 Folds et al.340/8 1. Filed Feb. 20, 1970 2,913,602 11/1959 Joy 340/8 L Paemed 1971 3239 801 3/i966 M Ga 11 340/8 L Assignee The United States oi Americans 1c ay represented by the Secretary of the Navy 1 Primary Examiner- RodneyD. Bennett, J r.

- Assistant Examiner-N. Moskowitz Attorneys-Richard S. Sciascia, Don D.Doty and William T. mrnERMAL ACOUSTIC LENS i2 Claims, 10 Drawing Figs.

ILLS. Cl 1181/.5, ABSTRACT: This invention is an acoustic refractor thatcon- 340/8 tains a combination of individual fluid lens elements havinglint. Ci 601v 1/16 different indices of acoustic refraction whichcooperate to Field of Search 340/8 C, 8 produce a uniform acousticrefraction over a wide range of LF, 5; 181/.5 ambient temperatures.

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7207 02? r are ("8/ PAIENTEDunv 1s ISYI 3,620 326 SHEET 3 BF 4 4/; i iii Z ERNEST A. HOGGE jg INVIL'NTOR.

PATENTEUuuv 16 I9?! FEGQD 9 E RNEST A. HOGGE INVENTOR.

ATI-IIEIRMAIL ACOUSTIC ILIENS STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF Til-IE INVENTION This invention pertains to the field ofacoustics. More particularly, the invention pertains to the acousticrefractor aspects of the acoustic arts, and in greater particularity,but not by the way of limitation, the present invention pertains to anacoustic lens which focuses acoustic energy impinging thereon. Ingreater particularity the invention as herein disclosed provides anacoustic lens which focuses acoustic energy impinging thereon to thesame focal plane over a wide range of temperatures. This lens isproperly described as being athermal because of this unique property.

A great many acoustic devices of the prior art employ a focused acoustictransducer to generate acoustic energy or to receive acoustic energy. Inmany such devices, a lens which is spatially separated from thetransducer is the focusing agent.

The lenses of the prior art which are most effective for underwaterapplications use a fluid refraction material which, because the soundvelocity therein is slower than in the ambient sea water, is formed tofollow the design practices of their optical counterparts. This resultsin a double convex lens which focuses impinging acoustic energy at apredetermined point on a focal plane determined by the geometricconsiderations of the location and bearing to the source of the acousticradiations and the focal length of the lens. Such lenses may be made tobe fixed focus and mounted together with the transducer in a unitaryassembly. However, difficulty is encountered when the unitsincorporating these lenses are operated in waters of temperatures otherthan that for which the lens was designed.

This temperature instability of the lenses of the prior art is due, insome part, to the fact that the propagation of the acoustic energythrough the medium and through the fluid lenses themselves is a functionof the temperature. Further, the gradient of temperature change is of anopposite sense so that of sea water, the most common operational medium.In such devices, the lens and transducer must be mounted in such afashion as to permit relative movement therebetween to correct the focuschanges brought about by the temperature variations when the operatingdevice is moved into different temperature waters. While the focus maybe corrected in this fashion, there are other faults introduced by thetemperature changes which cannot be so easily corrected.

SUMMARY OF THE INVENTION This invention overcomes the deficiencies oftheprior art by providing a lens which is uneffected by changes oftemperature over the normal temperature range encountered in the earthsoceans. This highly desirable result is obtained by employing a lenscomprised of a plurality of lens components made of solutions ofinorganic salts separated by acoustically transparent septa made ofrubber or the like. Because the propagation velocities in the componentlens elements are greater than velocities in the surrounding medium, thelens components of a positive lens combine to form shorter transmissionpaths on its axis than along its marginal positions. The lenses may useaspherical elements, as well as the more common spherical andcylindrical elements. The lens may, if desired, employ a metallicmounting with relatively good thermal conduction and an anechoic layeron the interior thereof. As will be explained herein, these individualfeatures cooperate in a new and improved fashion to produce a lens ofexceptional performance not heretofore obtainable with the prior artconstructions.

It is therefore an object of this invention to provide an acousticrefractor which is substantially uneffected acoustically by changes inthe temperature thereof.

A further object of this invention is to provide an improved acousticlens with uniform thennal response characteristics.

A further object of this invention is to provide an acoustic lens withfluid components, including at least one component comprised of anaqueous solution of inorganic salts.

Another object of this invention is to provide a compound acoustic lenshaving components with a faster sound propagation velocity than that ofthe ambient: medium.

A still further object of this invention is the provision of an improvedconstruction for an acoustic lens comprised of a plurality of individualcontiguous fluid components.

Yet another object of this invention is the provision of an acousticlens which exhibits a very small change of focus over a wide temperaturerange.

Other objects and many of the attendant advantages will be readilyappreciated as the subject invention becomes better understood byreference to the following detailed description, when considered inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a graphic representation ofthe temperature variations and the effect on the velocity of acousticpropagation for fluid lens substances of the prior art;

FIG. 2 is a graphic representation of the effect on acoustic propagationvelocity of increasing the molarity of an inorganic salt solutiontransmitting acoustic energy;

FIG. 3 is a graphic representation of the effect of temperature on theacoustic propagation velocity in solutions of potassium and sodiumchloride, as compared with sea water;

FIG. 4 is a graphic showing of the effect of temperature on therefractive index of a sodium chloride solution and a common organic lenssubstance used in acoustic lens construction;

FIG. 5 is a cross section of a concave lens structure according to theinvention;

FIG. 6 is an exterior view of a simple converging lens of sphericalsurfaces;

FIG. 7 is an exterior view of a simple converging lens of cylindricalsurfaces;

FIG. 8 illustrates, in longitudinal section, an alternative arrangementof the invention with a single refracting surface and an internal focalpoint;

FIG. 9 is a longitudinal section of a three element acoustic lensaccording to the invention; and

FIG. 10 is a longitudinal section ofa five element acoustic lensaccording to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of thepresent invention and the profound improvements made possible therebymay be more readily understood by reference to the graph of FIG. I. Asshown, the velocity of sound in sea water, as represented by plot 11,and in the common component substances of acoustic lenses, asrepresented by plots l2, l3, l4, and I5, is a linear function oftemperature. However, it should be noted that the slope of plot 11 is ofa different sign or direction than that of plots 12-15 representingmethanol, atlhanol, n-propanol. and n-butanol, respectively.

The amount that an acoustic energy wave is refracted, i.e.. the anglethat the direction of propagation is bent from the direction of travel,when passing from one medium into another, is determined by the index ofrefraction between the two mediums. This relates to the relativeacoustic propagation velocities of the two mediums according to thefollowing equation:

sin ti /sin 0 =C,/C (l) where 9, is the angle made by the impingingenergy wave in said first medium with respect to a normal to theinterface between the two mediums at the point of interception by theenergy;

e is the angle made by the radiation in said second medium with respectto a normal to the interface between the two mediums at the point ofinterception by the energy;

C is the velocity of propagation in the first medium; and

C,is the velocity of propogation in the second medium.

From the above equation, it may be seen that the temperaturecharacteristics of the organic compounds permit a lens made therefrom tofunction only over a relatively narrow range of temperatures. Further,since the lens is made from a relatively pure chemical, the index ofrefraction between adjacent mediums at a given temperature is dependentupon the particular materials involved.

Referring now to FIG. 2, there is shown a graphic representation of theeffect on the propagation velocities brought about by changing theconcentration of inorganic salts in solution. Curve I6 represents theaqueous solution of magnesium sulfate. Similarly, curve 17 correspondsto aqueous solutions of magnesium chloride and curves 18, I9, 211, and22 correspond to aqueous solutions of calcium chloride, sodium chloride,potassium chloride, and sodium bromide respectively. As shown by theillustrated curves, the sound propagation velocities in the aqueoussolutions of inorganic salts may be controlled by adjusting theconcentration of the inorganic salt. This permits the index ofrefraction of a lens made therefrom to be adjusted over usefully wideranges to produce the desired refraction from a particular configurationas might, for example, be dictated by an adjacent element. This abilityto tailor the index of refraction to the particular curve is animmensely useful design attribute of the structure of the invention.

The practicable limit on the amount of salt to be used in the aqueoussolution is imposed by the lower temperature limits at which the deviceis to be operated. That is, a molarity must be chosen such that thesolution is unsaturated at the lowest anticipated temperature to preventsolid granules of the salt from precipitating out of solution within thelens element. For sodium chloride, four molar solutions have worked wellin developmental studies. This concentration provides an optimumcompromise between a high index of refraction and freedom from becomingsaturated at lower temperatures.

Referring to FIG. 3, curve 23 represents the velocity of sound in seawater as a function of temperature, while curves 24 and 25 show similarcharacteristics of 4-molar solutions of potassium chloride and sodiumchloride. As indicated by curves 23-25, the velocities of propagation inthe two illustrated saline solutions are greater than the correspondingvelocity of sound in sea water of the same temperature. Likewise, it maybe noted that the rate of change and direction of change areapproximately the same as for sea water. This stands in marked contrastto the organic refractive materials of the prior art, as illustrated inFIG. l and discussed above.

The marked difference in the thermal effect on the propagationvelocities is shown in the index of refraction variations illustrated inFIG. d. Curve shows the variation of the index of refraction of a fourmolar solution of sodium chloride with respect to sea water. Curve showsa much greater change for Heptacosafluortributylamin (C F N), anothercommon organic material used in prior art acoustic lenses. Most otherorganic substances show an even greater variation of index ofrefraction.

It should be noted that mixtures of inorganic salts may be used to goodeffect in the practice of applicants invention. Such mixtures may beused to produce a desired index of refraction or other desirableproperties. The exact proportions at which the various saline solutionsare combined to produce the desired effects may be arrived at bysuitable experimentation, since the developmental studies indicate thatthe exact properties of a solution of a mixture of salts are difficult,at the present time, to accurately predict. In this regard the mixing ofsalts in aqueous solution is similar to the metallurgical arts in beingsomewhat empirical. The results obtained by the various saline mixturesare reproducible with similarly constituted mixtures so a uniformity ofproduct may be assured. It would be appear that the acoustic propertiesof saline solutions follow, in a complex fashion, the thermodynamicproperties. For purposes of illustration, only solutions of a singlesalt will be considered.

Because the velocity of propagation of sound is greater in the inorganicsalt solutions, a concave lens surface concentrates or focuses,impinging acoustic energy. Lenses made according to the invention aredistinguished by being concave rather than the familiar convexstructural arrangements of the prior art organic elements. An example ofthis construction is shown in FIG. 5.

Referring to FIG. 5, the housing 26 has concave acoustically transparentmembranes 27 and 28, which may be made of synthetic rubber, for example,at opposite ends thereof. The interior space between membranes 27 and 28is filled with a suitable saline solution 29. In the exampleillustrated, saline solution 29 is of a sodium chloride solution of fourmolar concentration. The interior of the housing 26 enclosing salinesolution 29 may be provided with an anechoic layer 31, if desired.Anechoic layer 31 minimizes lens distortions and aberrations caused byacoustic energy being reflected from the inner surface of housing 26.The length of housing 26 is determined by the radii of membranes 27 and28, the axial spacing illustrated as the dimension T thereof, and theaperture desired. In the example of FIG. 4, membranes 27 and 28 arehemispherical, providing the maximum aperture for a given radius ofcurvature, and the length of housing 26 is r,+ r +T the thickness ofmembranes 27 and 28. I! should be understood that membranes 27 and 28may comprise smaller portions of spherical surfaces, i f desired. Thecombined thickness of membranes 27 and 28 is normally so small that i!may be disre garded. However, in some constructions, for example, wherethe ability to withstand great pressures and shock waves is required,the thickness of the membranes may become significant. Even in theinstances where the membrane thickness is considerable, the lens 0 f theinvention retains its improved thermal properties. Of course, theindividual parameters of constructions may be varied to produce a lenshaving the desired focal length and energy gathering ability inaccordance with the practices of good acoustic lens design. In thisrespect, the radii of membranes 27 and 28 may include valves between 2.5cm. and 1.0 m. and the separation T therebetween may include appropriatevalues between 0.5 mm. and 10 cm. Likewise, the lens may be made withdifferent radii for membranes 27 and 28 to produce an asymetric lensrather than the symmetrical arrangement with equal radii illustrated forpurposes of explanation at FIG. 4. Such modifications of the illustratedinvention are considered obvious expedients to a person skilled in theart.

The operation of the lens of FIG. 5 may be better understood byconsidering the path of a marginal ray 32 of acoustic energy impingingparallel to the acoustic axis 33. Ray 32 is refracted toward the centralacoustic axis 33 of the lens. Ray 32 impinges membrane 28 at an angle 9,and is refracted in saline solution 29 to exit membrane 28 at an angle0g given by the equation:

sin 0 =sin {,Q/C, (2) where C is the propagation velocity in the ambientmedium; and

C is the propagation velocity in the saline solution.

Ray 32 impinges on membrance 27 at an angle 0 which may be determined bygeometric considerations including the thickness or membrane spacing T.Ray 32 is again refracted through angle 0., in passing through membrane27 to cross the acoustical axis 33 at focal point 34. The angles 0, maybe determined in a similar manner to that used to determine 0,, payingspecial attention to the fact that the index of refraction changes asthe ray passes from the region of high propagation velocity to arelatively slower one, assuming that the lens is immersed in a uniformmedium. In one embodiment solution 29 is a 20 percent aqueous solutionof sodium chloride, r and r are each 3 cm., axial thickness T is 2 cm.and, in 15 C. sea water, the lens so constructed has a focal length of6.625 cm.

The lens of the invention may be comprised of surfaces of revolution, ifdesired, as shown in FIG. 6. In such an instance, the membranes 27 and28 are spherical surfaces and the lens housing 26 is cylindricallyshaped. Such a lens is desirable when the desired beamwidth is narrowand the transducer with which the lens may be used is of small physicaldimensions.

Referring to FIG. 7, a lens 36 is shown to be constructed in a finalconfiguration that is useful when employed in combination with elongatedtransducers. In this embodiment the membranes 2'7 and 28 lie alongsurfaces which may be considered as having been generated by moving thegenerating curve along a line or plane at right angles to the acousticalaxis to produce a lens 36 comprised by cylindrical refracting surfacesand having a generally elongated housing. The ends or lateral marginalportions 37 of lens 36 may be rounded and the refractive surface in thethese areas spherical, as in lens 35, if desired. Alternatively, theends 37 may be left squared, if desired.

Both lenses 35 and 36 are designed to be immersed in an ambient medium,such as sea water or the like, and positioned in cooperativerelationship to a suitable acoustic energy transducer, now shown. Tothis end, they may be made as part of an acoustically opaque housingcontaining the transducer, in a manner similar to the manner in which anoptical lens is mounted on the lens board of a camera body.

FIG. 8 illustrates an alternative arrangement of the invention. Ahousing 38 provides an outer cover for the lens and supports, at one endthereof, a membrane 33. An anechoic layer il serves the same function aslayer 31 of the lens of FIG. t and is supported on the interior ofhousing 38. The interior of the lens is filled with a saline solution 62of a predetermined composition which, for purposes of explanation andcompleteness, may be a 43 percent solution of calcium chloride.

Membrane 39 is curved such that a marginal ray d3 impinging thereon isrefracted to a focal point did on acoustical axis 45. Focal point 6 iswithin the confines of housing 3! This permits the appropriatetransducer to be mounted within the protective confines of housing 36and results in a compact, economical construction but with a singlerefracting surface. Like the lens of FIG. 3, the lens shown in sectionof FIG. 8 may be made with the refracting surface either of a sphericalor ofa cylindrical configuration.

Referring to FIG. 9, a longitudinal section of a triple lens accordingto the invention is illustrated. Lens housing 46 supports acousticallytransparent membranes &7, d8, 69, and 511 to effectively produce threerefracting elements 62, 53, and 54 in the volume enclosed therebetween.The elements 52, 53, and 5d cooperate to bring an impinging acousticenergy beam 55 to a focus at focal point 56 on acoustic axis 57. As inthe lenses of FIGS. 4 and 7, anechoic layers 56 and 59 may be providedon the periphery of elements 52 and 54, if desired.

As shown, membranes 48 and 6'9 may have a combined axial thickness andradii of curvature such that refractive element 33 enclosed thereby doesnot extend completely across the full aperture of housing 46. As isunderstood by those familiar with lens design, refracting element 53need extend only to the extent necessary to intercept the marginal raysaccepted by refractive element 52. Beyond the critical size where thistotal interception occurs, the desired thickness of element 63 is thecontrolling parameter for the lateral extend of elements of this type.

Membranes A8 and 49 are acoustically transparent and must be opaqued intheir noncurved, or plane, portions 6ll to prevent transmission ofenergy directly between elements 52 and 54 without intermediaterefraction by lens element 53. This opaque septum may be provided byextending the anechoic layer of each of refracting elements 52 and 54over the plane portions 61 of to produce a stop surrounding refractiveelement 33.

As illustrated in FIG. 3, there are four refractions, or bendings, ofthe acoustic beam 55 as it traverses the lens. The angles of refractionare designated as 6 0 and 6 in the illustration while the correspondingangles of incidence are designated as 9,, 6 0 and 0 The relationshipbetween the respective angles is as given by equations (1) and (2)above. The refracting elements 52 and 56 may contain the same salinesolution, if symmetrical construction is desired. Alternately thecomposition of the solutions, or the concentration thereof, may bedifferent for elements 52 and 36, if an asymmetric construction isdesired. Refracting element 33 is made of a material selected to have aslower acoustic propagation velocity than elements 32 and 54.

Like the previous examples of lens construction described above, theradii of curvature for membranes 6?, d6, d3, 51, identified as n, r rand r respectively, are chose to produce a lens having the desired focallength and energy gathering properties. Although a wide variation invalues for these parameters is possible, the respective radii lie withinthe ranges indicated below.

The negative sign is indicative of the direction of curvature beingconcave in the direction of energy transmission.

By way of more specific example of the preferred construction, a lens ofthe configuration illustrated in FIG. 3 having a focal length of 273.82mm. in fresh water at 22.5 C. has front and rear elements 52 and 56 madeof 26 percent aqueous solution of NaCl which has a sound propagationvelocity of 1784.6 m./sec. Central element 33 is made of fresh waterhaving a sound propagation velocity of 11483.6 m./sec. The radii ofcurvature of membranes d7, 66, 69', and 311, identified as in theforegoing example by r,, r r and r.,, respectively, are as follows:

r =l 27.00 mm.,

r,,=88.90 mm.

Where the radii of curvature of the membranes become small, such thatthe angles of incidence become large, undesirable reflections from themembrane surfaces may occur. These reflections, analogous to thephenomenon of flare in the optical lens, degrades the image qualityproduced by the lens. These reflections, and the acoustic imagedegradation produced thereby, may be minimized by providing furtherrefracting elements of intermediate refracting power. Such aconstruction is illustrated at FIG. 10.

FIG. 9 illustrates a seven-element athermal acoustic lens comprised bysaline solution fluid elements 62, 63, 64, 65, 66, 67, and 68 housedwithin a common lens housing 69. The end portions 7ll of housing 69extend partially over the aperture to provide stops to include energyfrom the planomarginal portions of refracting element 63. Acousticallytransparent membranes 72, 73, 74, 75, 76, 77, 78, and 79 comprise thecontaining structure to separate the solutions of refracting elements62, 63, 6d, 65, 66, 67, and 63 for mixing with each other and thesurrounding fluid, as well as defining the shape of the respectiveelements. Anechoic layers 81 and 82 on the interior walls of housing endportions 71 prevent acoustic reflections from these surfaces. Anechoiclayers 63 and 34 line the interior surface of that portion of housing 69occupied by elements 66 and 66. Anehoic layers 33 and 36 also cooperatewith plane portions 86 and 86 of membranes '75 and 76 to provide aninternal stop in the same fashion as anehoic layers 53 and 59 in theembodiment of FIG. 9.

The lens of FIG. I6 may, like the previously described exemplaryconstructions, be constructed to provide the desired focal length byaltering the composition of the elements and their dimensions. Thegreater number of elements present in the construction of FIG. It)permit a more precise compensation of thermal and refractive properties.For example, elements 62, 65 and 68 may be of the same acousticpropagation velocity as the intended surrounding medium, e.g., fresh orsalt water. Refracting elements 63 and 67 would be made of a fluid withan intermediate propagation velocity such as a 20 percent solution ofsodium chloride. Elements 64 and 66 are made of a fluid with a highvelocity of propagation such as a saturated solution of sodium chloride.

As a further example of the flexibility of manipulation of the acousticparameters afforded by the construction of FIG. 10, it should be notedthat the elements 63 and 67 may be designed to eliminate reflections atthe interfaces between elements 62 and 64, and 66 and 68. This ispossible if the composition and solution concentration of the twointermediate elements 63 and 67 is chosen such that the acousticimpedance thereof is equal to the square root of the product of theacoustic of the two adjacent elements 62 and 641 and 66 and 68. ln suchan antireflection construction the thickness of elements 63 and 67 ischosen to be an odd number of quarter wavelengths in the material fromwhich the elements 63 and 67 are made. In this particular, elements 63and 67 are somewhat analogous to the antireflection coatings so wellknown in the optical lens art.

The foregoing exemplary embodiments of applicants invention are capableof modification to produce the particular focal lengths and acousticimaging properties desired. It should be understood that the membranesused to separate the fluid lens components are very thin and are shown,herein, with their thickness exaggerated for purposes of explanation.Likewise, the lenses are illustrated as being symmetrical, but, ifdesired, they may be made asymmetrical.

The foregoing description taken together with the appended claimsconstitute a disclosure such as to enable a person skilled in theacoustic arts and having the benefit of the teachings contained thereinto make and use the invention. Further the structure herein describedmeets the objects of invention, and generally constitutes a meritoriousadvance in the acoustic art unobvious to such a skilled worker nothaving the benefit of the teachings contained herein.

Obviously, other embodiments and modifications of the subject inventionwill readily come to the mind of one skilled in the art having thebenefit of the teachings presented in the foregoing description and thedrawings. It is, therefore, to be understood that this invention is notto be limited thereto and that said modifications and embodiments areintended to be included within the scope of the appended claims.

What is claimed is:

1. An athermal acoustic lens comprising in combination:

housing means having an aperture extending therethrough and acousticallyopaque wall portions surrounding said aperture;

membrane means attached to said housing means and extending across saidaperture to provide a fluidtight volume within said housing means, saidmembrane being acoustically transparent and having predeterminedgeometrical configurations; and

a solution of inorganic salts filling said fluidtight volume in suchmanner as to effect an acoustic refractor means thereat.

2. An athermal acoustic lens according to claim 1 in which said solutionof inorganic salts has an acoustic propagation velocity which is higherthan that of sea water.

3. An athermal acoustic lens according to claim 1 wherein said solutionof inorganic salt is an aqueous solution of sodium chloride.

4. An athermal acoustic lens according to claim ll further comprising alayer of anechoic material lining the inner surface of said housingmeans within the confines of said fluidtight volume.

5. An athermal lens according to claim )1 wherein said membrane meanshas a spherical geometric configuration.

6. An athermal acoustic lens according to claim 1 wherein said membranemeans comprises a plurality of acoustic septa separating said housingmeans into a plurality of contiguous fluidtight volumes.

7. An athermal acoustic lens according to claim 6 wherein at least oneof said fluidtight volumes is filled with a fluid so constituted as tocooperate with the shape thereof to eliminate surface reflectionsbetween volumes adjacent thereto.

8. An athermal acoustic lens according to claim 6 wherein adjacent onesof said fluidtight volumes are filled with different aqueous solutionsof inorganic salts. I

9. An athermal acoustic lens according to claim 7 wherein the pluralityof septa are four in number, so as to enclose three reflective elementstherebetween, wherein said refractive elements and said septa arenumbered consecutively from front to rear of said athermal acousticlens, and wherein said septa have radii of curvatures identifiedconsecutively from front to rear of the lens as r,, r r and r.,, withsaid radii having values within the ranges indicated in the table:

60 mm. r 1.6 m.,

25 mm. r l m., wherein the minus sign indicates that the center ofcurvature of the radius of curvature identified thereby lies on theentrant side of said septum.

10. An athermal lens according to claim 8 wherein the entrant and exitrefracting elements, between the first and second septa and between thethird and fourth septa, respectively, are comprised of an aqueoussolution of sodium chloride and the central refracting element, betweenthe second and third septa, is comprised of distilled water.

111. An athermal acoustic lens according to claim 9 wherein said centralrefracting element is marginally surrounded by an acoustically opaquestop.

12. An improved acoustic lens for use within fluid mediums to refractacoustic energy comprising in combination:

housing means having opaque wall portions;

aperture means extending through said housing means for the passage ofacoustic energy therethrough along an acoustic axis;

membrane means attached at the outer periphery thereof to said housingmeans to extend across said aperture means, said membrane means beingmade of acoustically transparent, fluidtight material and ofpredetermined geometrical configuration, so as to provide at least onefluidtight volume within said housing means; and

an aqueous solution of an inorganic salt filling said fluidtight volumeso as to provide an acoustic refractive element thereat, said inorganicsalt being selected to have a ther mal change of acoustic propagationvelocity such that the effective focal length of the lens remainssubstantially unaltered over the range of temperature normallyencountered in the oceans and seas of the earth.

a s a a UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,620,326 Dated November 16, T97] Inventor(s) Ernest A. Hogge It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 2, line 58, the word "athanol" should read --ethanol--.

Column 2, line 67, equation (l) should read sin 6 /sin 9 C /C Column 5,line l8, the word now" should read --not--.

Column 6, line 9, the word "chose" should read --chosen--.

Column 7, line 9, between the words "acoustic' and "of", the word--impedances-- should be inserted.

Signed and sealed this 20th day of June 1972.

(SEAL) Attest:

EDWARD M.FLEI'CHER ,JR. ROBERT GO'I'TSCHALK Attesting OfficerCommissioner of Patents Column 4, line 53, equation (2) should read sin9 sin e (C /C )RM pomso 1 USCOMM-DC 60376-969 U 5 GOVERNMENY PRINYINGOFFlCE (Q69 0 355 33

1. An athermal acoustic lens comprising in combination: housing meanshaving an aperture extending therethrough and acoustically opaque wallportions surrounding said aperture; membrane means attached to saidhousing means and extending across said aperture to provide a fluidtightvolume within said housing means, said membrane being acousticallytransparent and having predetermined geometrical configurations; and asolution of inorganic salts filling said fluidtight volume in suchmanner as to effect an acoustic refractor means thereat.
 2. An athermalacoustic lens according to claim 1 in which said solution of inorganicsalts has an acoustic propagation velocity which is higher than that ofsea water.
 3. An athermal acoustic lens according to claim 1 whereinsaid solution of inorganic salt is an aqueous solution of sodiumchloride.
 4. An athermal acoustic lens according to claim 1 furthercomprising a layer of anechoic material lining the inner surface of saidhousing means within the confines of said fluidtight volume.
 5. Anathermal lens according to claim 1 wherein said membrane means has aspherical geometric configuration.
 6. An athermal acoustic lensaccording to claim 1 wherein said membrane means comprises a pluralityof acoustic septa separating said housing means into a plurality ofcontiguous fluidtight volumes.
 7. An athermal acoustic lens according toclaim 6 wherein at least one of said fluidtight volumes is filled with afluid so constituted as to cooperate with the shape thereof to eliminatesurface reflections between volumes adjacent thereto.
 8. An athermalacoustic lens according to claim 6 wherein adjacent ones of saidfluidtight volumes are filled with different aqueous solutions ofinorganic salts.
 9. An athermal acoustic lens according to claim 7wherein the plurality of septa are four in number, so as to enclosethree refractive elements therebetween, wherein said refractive elementsand said septa are numbered consecutively from front to rear of saidathermal acoustic lens, and wherein said septa have radii of curvaturesidentified consecutively from front to rear of the lens as r1, r2, r3,and r4, with said radii having values within the ranges indicated in thetable: -22 mm. <r1<-1 m., 60 mm. <r2<1.6 m., -60 mm. <r3<-1.6 m., 25 mm.<r4<1 m., wherein the minus sign indicates that the center of curvatureof the radius of curvature identified thereby lies on the entrant sideof said septum.
 10. An athermal lens according to claim 8 wherein theentrant and exit refracting elements, between the first and second septaand between the third and fourth septa, respectively, are comprised ofan aqueous solution of sodium chloride and the central refractingelement, between the second and third septa, is comprised of distilledwater.
 11. An athermal acoustic lens according to claim 9 wherein saidcentral refracting element is marginally surrounded by an acousticallyopaque stop.
 12. An improved acoustic lens for use within fluid mediumsto refract acoustic energy comprising in combination: housing meanshaving opaque wall portions; aperture means extending through saidhousing means for the passage of acoustic energy therethrough along anacoustic axis; membrane means attached at the outer periphery thereof tosaid housing means to extend across said aperture meanS, said membranemeans being made of acoustically transparent, fluidtight material and ofpredetermined geometrical configuration, so as to provide at least onefluidtight volume within said housing means; and an aqueous solution ofan inorganic salt filling said fluidtight volume so as to provide anacoustic refractive element thereat, said inorganic salt being selectedto have a thermal change of acoustic propagation velocity such that theeffective focal length of the lens remains substantially unaltered overthe range of temperature normally encountered in the oceans and seas ofthe earth.